Auxiliary power units (“APU”s) are gas turbine engines that are used in aircraft to provide electrical and pneumatic power for various equipment therein. Typically, APUs are located in the aft section of the aircraft, such as in the tail cone, and are isolated from other sections of the aircraft by a firewall. During operation, APUs may produce exhaust gas. In some aircraft, the exhaust gas is directed through a nozzle and an exhaust duct, and out the aircraft through an exhaust opening. In some aircraft configurations, the APU nozzle may communicate with an eductor system that uses the APU exhaust gas to draw and direct other gases through the aircraft.
Many eductor systems include a dual plenum configuration. For example, in some systems, a first plenum is used to draw gas across an oil cooler (referred to herein as the “oil cooler plenum”), while a second plenum is used to direct surge flow gas to the exhaust duct. The two plena are typically tandemly arranged. In one conventional configuration, the APU nozzle is surrounded by the two tandemly arranged plena, and the oil cooler plenum is placed in a location forward of the surge flow plenum. In another conventional configuration, the oil cooler plenum is placed aft of the surge flow plenum. In still another conventional configuration, the oil cooler plenum circumscribes the APU nozzle and the surge flow plenum is disposed downstream of the oil cooler plenum and APU nozzle.
Although tandemly-arranged eductor systems, such as those described above, are useful, they do suffer from certain drawbacks. For example, these tandem plena may not fit within converging aircraft tail cones, or “blade tails”, which have recently been incorporated into new aircraft designs to lower operating costs and increase aircraft fuel economy. Specifically, tandem plena may be too long and bulky. to be placed in these blade tails and may interfere with the walls of the blade tail. To better accommodate the plena, some designs move the APU forward to place the plena in a wider portion of the tail cone. However, these configurations may not be desirable because the distance between the forward fire wall and the APU may not be sufficient to allow removal of various line replacement units (“LRUs”) during repair and overhaul of other tailcone components.
Hence, there is a need for an oil cooler and surge flow plenum system that may be disposed in a blade tail. Additionally, it is desirable for the system to be lightweight and relatively compact. Moreover, there is a need for a system that, when installed, provides a sufficient distance between the forward firewall of the tailcone and the APU to allow removal of LRUs during repair and overhaul of other tailcone components.